1. Field of the Invention
The present invention relates to a foamed rotary member which is used as a paint roller, a paper feed roller, a cleaning roller, or the like, and a method of manufacturing the same and, more particularly, to a toner supply rotary member and a developing device using the same.
2. Related Background Art
Various polyurethane foamed rollers that use a polyurethane soft slab foam used in industrial materials, industrial products, and the like are manufactured in such a manner that a prism member having required dimensions is cut out from a soft slab foam block (foamed raw foam) by various cut-out processing methods, and the prism member is subjected to a cylindrical process such as surface polishing so as to have a roller shape.
The polyurethane soft slab foam block is manufactured by open foaming on a foaming conveyor, and cells in the foam structure have shapes approximate to that of an elliptic member vertically extending along the foaming direction.
The manufacturing method of the foam block will be explained below with reference to FIGS. 4A to 4D.
FIG. 4A shows a foamed raw foam which is elongated in the direction of an arrow in FIG. 4A by foaming urethane in a mold having a length corresponding to the roller length that requires a predetermined depth.
A soft slab foam block 4 as the foamed raw foam shown in FIG. 4A is cut in a direction perpendicular to the foaming direction indicated by the arrow in FIG. 4A to have an appropriate thickness corresponding to the required diameter of the roller (the cutting process in the direction perpendicular to the foaming direction is generally called "a traverse process") to obtain a planar member 7 extending in the direction perpendicular to the foaming direction, as shown in FIG. 4B. The obtained planar member 7 is cut in the foaming direction (the cutting process in the foaming direction is generally called "a longitudinal process") to obtain a prism member 8 in the direction perpendicular to the foaming direction, as shown in FIG. 4C. The prism member 8 is cut to have a required length if necessary. Thereafter, the prism member 8 is subjected to a cylindrical process by means of a polishing process using a grinder or a cutting process using a heating metal wire such as a nichrome wire or the like, and is then hollowed out to a predetermined shape by inserting a rotating cylindrical blade, as needed, thus manufacturing a roller 3 having a required diameter, as shown in FIG. 4D.
As the cut-out processing method of the prism member 8 from the planar member 7, a method of cutting a foam block using a heated nichrome wire, a method of punching out a foam block using a press machine attached with a mold formed by a Thomson blade, and the like are available.
When urethane is foamed in the mold, each cell has a prolate spheroid shape which is long in the foaming direction.
The urethane foamed roller 3 obtained by the manufacturing method shown in FIGS. 4A to 4D has a roller axis direction according with the direction perpendicular to the foaming direction since the slab form block 4 is traverse-processed and is then longitudinal-processed to obtain the prism member 8, and the prism member 8 is processed to have a cylindrical shape. For this reason, each cell in the foam is formed into an elliptic shape extending in the direction perpendicular to the roller axis direction.
Therefore, the sections (ellipses), in the major axis direction, of elliptic cells 2 appear at a given position on the outer circumferential surface of the urethane formed roller 3, as shown in FIG. 2C, and the sections (circles), in the minor axis direction, of elliptic cells 2 appear at a position rotated through 90.degree. from the former position in the circumferential direction, as shown in FIG. 2A. At a position between these positions, cells 2 have slightly different sectional shapes and sizes since their sectional shapes are changing from an ellipse to a circle or vice versa, as shown in FIG. 2B.
For this reason, the recess portions on the surface of the roller 3 have uneven shapes and volumes. As a result, not only the hardness but also the holding ability of, e.g, a paint of the roller 3 change in the circumferential direction of the surface of the roller 3.
In the conventional urethane foamed roller 3, not only the surface but also the interior are uneven in the circumferential direction, and the physical properties such as the hardness, modulus of elasticity, and the like are not constant in the circumferential direction.
For this reason, the surface of the roller is not easy to polish. In addition, when the roller is used as a paper feed roller or cleaning roller, the roller may locally slip or cause a cleaning error in the circumferential direction.
FIG. 5 shows a developing device using a toner supplying roller.
The developing device comprises a developing chamber 102, which has an opening portion 103 opposing a photosensitive drum 101 serving as an electrostatic latent image carrier, and stores toner. A developing sleeve 110 is arranged in the developing chamber 102, and serves as a toner carrier, which carries the toner and conveys the toner to the photosensitive drum 101.
The developing sleeve 110 is arranged in the developing chamber 102 so that a portion of its outer circumferential surface externally protrudes from the opening portion 103. The developing sleeve 110 is held to be separated from the photosensitive drum 101 by a gap of 50 to 500 .mu.m, and is formed with a developing area to supply the toner carried on the developing sleeve 110 toward the photosensitive drum 110. Also, a toner supplying roller 112 for supplying the toner conveyed by a convey means 111 to the developing sleeve 110 is arranged in the developing chamber 102.
The developing sleeve 110 is applied with a developing bias obtained by superposing an AC voltage on a DC voltage. The developing bias is generated by a bias power supply 106.
A blade 113 for regulating the thickness of a toner layer carried on the developing sleeve 110 is arranged above the developing sleeve 110. The blade 113 is attached to the developing chamber 102. A blow-out prevention sheet 108 for preventing toner from externally blowing out from the lower portion of the developing chamber 102 is arranged below the developing sleeve 110.
Upon development, the convey means 111 conveys toner toward the toner supplying roller 112, and the toner is applied onto the developing sleeve 110 by the toner supplying roller 112. The developing sleeve 110 is rotated in the direction of an arrow in FIG. 5, and the toner carried on the developing sleeve 110 is regulated by the blade 113 to have a predetermined toner layer thickness. Thereafter, the toner is supplied to the above-mentioned developing area. On the developing area, an electric field is formed by the developing bias, and the toner on the developing sleeve 110 flies toward a portion, where a latent image is formed, of the surface of the photosensitive drum 110 due to the presence of the electric field force.
In the above-mentioned developing device, both magnetic and nonmagnetic toners can be used. Since magnet or the like is dispersed in a resin such as stylene, acryl, or the like in magnetic toner, if color toner is formed by magnetic toner, only dull colors with low saturation will be obtained. Therefore, when a color image is formed using monocomponent developing agents, nonmagnetic toner that does not contain any magnetic member is preferably used.
The toner supplying roller 112 and the developing sleeve 110 rotate in the same direction, and the toner supplying roller 112 peels off the residual toner on the developing sleeve 110. If the residual toner is insufficiently peeled off, highly charged toner by triboelectrification remains on the surface of the developing sleeve 110. When the highly charged toner is deposited upon rotation of the developing sleeve 110, it prevents new toner from contacting the developing sleeve 110, and new toner cannot be sufficiently charged.
As a result, both insufficiently charged toner and the charged-up toner are present on the developing sleeve to form a broad charge distribution, thus causing errors such as fogging or low density due to insufficiently charged toner, a blotch (a phenomenon that toner does not attach in a sufficient amount to the developing portion) due to highly charged toner, and the like. Furthermore, a ghost phenomenon is induced, i.e., the previously formed image is again formed for about one revolution of the developing sleeve.
In the case of magnetic toner, a magnetic constraint force can act on the lower charged toner by triboelectrification since the developing sleeve 110 contains a magnet, thus easily preventing fogging. However, in the case of nonmagnetic toner, since most of the forces acting on the toner are almost by electrostatic forces, fogging caused by low charged toner considerably occurs.
As an elastic member of, e.g., a sponge constituting the toner supplying roller 112, foamed urethane is often used. In the foamed urethane, the material grows upward as it is foamed, and cavities F and walls G form continuous cells, as shown in FIG. 10.
When the toner supplying roller is formed using such foamed urethane raw foam, as shown in FIG. 6, a raw foam C is divided so that the lateral direction perpendicular to the foaming direction of the raw foam C accords with the longitudinal direction of the roller, similarly to the example shown in FIGS. 4A to 4D. A shaft L is inserted into a divided foamed urethane D, and the surface of the foamed urethane D with the shaft L inserted is processed to complete a roller E, i.e., the toner supplying roller 112. With this process, the toner supplying roller 112 can be formed efficiently without wasting an expensive raw foam.
However, since cells have different shapes in their sections in the longitudinal and lateral directions with respect to the foaming direction, if the lateral direction of the raw foam is the longitudinal direction of the roller, the toner peeling ability and the toner supplying ability of the toner supplying roller 112 change every 90.degree. in the circumferential direction of the roller. As a result, the image density changes or fogging occurs at a half revolution pitch of the toner supplying roller.
In the case of a cartridge type developing device, in order to prevent toner from leaking before use, the developing chamber and a toner storage chamber are partitioned, and the partition is removed when the device is used, so that toner can be supplied first to the toner supplying roller 112. For this reason, the unevenness, in the circumferential direction, of the roller leads to an unevenness in the amount of toner conveyed. The density unevenness tends to continue until toner attaches well to the toner supplying roller, i.e., until toner particles enter the cavities on the surface of the toner supplying roller and the roller surface uniformly carries the toner.
Furthermore, since the contact pressure of the toner supplying roller 112 with respect to the developing sleeve 110 changes depending on the concentration of cells, the driving torque of the developing device varies to cause pitch unevenness on an image. Also, the toner supplying roller is separated from the developing sleeve every half revolution due to a repulsion force and is vibrated, thus causing pitch unevenness. In addition, the toner supplying ability and the peeling ability change, resulting in image unevenness.
In recent image forming apparatuses, since images have higher resolutions, image unevenness such as pitch unevenness is more conspicuous. Especially, in a color image forming apparatus which forms a full-color image by superposing color images a plurality of number of times, the unevenness may appear as a color difference or may be emphasized by color superposition.